Extract interfaces from your classes and put them into a core project referenced from both Billing and Financial projects. You can then use those interfaces to share data between assemblies.
This only allows you to pass objects between those 2 assemblies, but you can't create objects from the other since you don't actually have a reference to begin with. If you want to be able to create objects you need a factory, external to those 2 projects, that handles object creation.
I would extract the business logic that needs to share the data back and forth between Billing and Financial into another project. This would make things a lot easier and would save you from resorting to all sort of tricks that make maintainability a nightmare.
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